IP switch, interface circuit and ATM switch used for IP switch, and IP switch network system

ABSTRACT

In a system in which communication is performed among a plurality of devices having a mechanism of disassembling a packet into ATM cells to send them and a plurality of devices having a mechanism of assembling the received ATM cells into the packet, a frame is prevented from being lost owing to change of virtual connections in the course of transferring the frame, improving reliability of the communication on the packet level. 
     When the switch receives a request for change of switching, it does not processes that request at once, but confirms that the cell located at the boundary of the frame has been processed, before processing the change request, so as to protect the frame. 
     When the switch receives a request for change of switching, it protects a frame by multicasting the cells to both destinations before change and after change, for a given period of time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims the benefit ofpriority to, Applicant's U.S. application Ser. No. 08/998,382, filedDec. 24, 1997, now U.S. Pat. No. 6,304,555 and from Applicant's JapanesePatent Application No. 08-344981, filed on Dec. 25, 1996, both of whichare incorporated by reference herein as fully as if set forth in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an IP switch (internet Protocol switch)for transferring packet data in the lower layer of ATM (AsynchronousTransfer Mode), to an interface circuit and an ATM switch used for thatIP switch, and to an IP switch network system.

2. Related Art Statement

At present, IP (Internet Protocol) is widely used as a protocol incomputer communication in which data is divided into packets on acomputer before transmission. According to the 7 layer model of OSI(Open System Interconnection), IP is the protocol which belongs to thenetwork layer, and, to perform communication between different physicalnetworks, communication is realized through routers.

However, it has been generally pointed out that it is difficult to makethe IP router fast and to provide it with mass storage because of thecharacteristics of IP, and accordingly, even when high-speed lowerlayers are employed, their capacity can not be utilized sufficiently.

Recently, as a solution to these problems, there has been noted atechnique called “IP Switching” proposed in Flow Labeled IP: AConnectionless Approach to ATM (Proc. IEEE Infocom, San Francisco, March1996). This technique is one that introduces the arrangement of ATM(Asynchronous Transfer Mode) known as a high-speed communicationtechnique into the ordinary IP, in an attempt to speed up the IP router.

SUMMARY OF THE INVENTION

In the above prior art, the process in the network layer, which ishandled by software, is simplified as much as possible, to speed up theprocess, and retransmission by the cell is not taken into consideration.Accordingly, even if only one cell is lost, the whole frame containingthat cell is discarded. As a result, when a virtual connection ischanged in the middle of transfer of a frame, that frame is lost fromthe network.

Conventionally, in ATM, it is provided that a virtual connection isestablished in advance, and data is transferred on that line,presupposing that the transfer line is not changed in the middle ofcommunication. Thus, there has not been such a problem as a packet beinglost owing to change of a transfer line during the course of transfer.

In the IP switching technique, however, since a virtual connection ischanged during the course of data transfer, it is possible that suchloss of a packet causes a problem. In the following, such an examplewill be described.

Conventionally, such packet loss has been dealt with by retransmissionupon timeout in the network layer. However, this processing usuallyoccurs at the time of a fault such as congestion or link breakage, andtoo frequent occurrence of this processing may give rise to congestion,or a transfer rate may be lowered by awaiting timeout. Thus, from theviewpoint of quality, it is problematic to use the timeoutretransmission technique for such a frequent phenomenon as change of avirtual connection in IP switching.

In particular, in the case that IP switching is used in a public networkwhich employs packet metering as an accounting method, the technique ofhandling packet loss relying on retransmission in the network layer cannot be accepted because double accounting on a user may arise.

An object of the present invention is to provide an IP switch, interfacecircuit and an ATM switch used for that IP switch, and an IP switchnetwork system, which can prevent loss of a frame caused by change of avirtual connection in the middle of frame transfer.

Another object of the present invention is to provide an IP switch,interface circuit and an ATM switch used for that IP switch, and an IPswitch network system, which can avoid retransmission owing to theabove-described frame loss caused by change of a virtual connection inthe middle of frame transfer, and can avoid double accounting on a userdue to retransmission of a packet.

Another object of the present invention is to provide an IP switch,interface circuit and an ATM switch used for that IP switch, and an IPswitch network system, which can avoid retransmission owing to theabove-described frame loss caused by change of a virtual connection inthe middle of frame transfer, and can improve the transfer rate.

In a first invention, when a virtual connection is to be changed, a PT(Payload Type) field of an ATM cell which is currently in the course oftransfer is referred to, and the transfer line is kept from beingchanged until the boundary of the frame is recognized, so that an AAL(ATM Adaptation Layer) frame is protected and packet loss is prevented.

In a second invention, when a virtual connection is to be changed, themulticast function of the ATM switch is employed to transfer asufficient number of cells for transmission of the maximum length ofpackets through both transfer lines before and after the change, andthereafter the virtual connection before the change is eliminated, sothat an AAL5 (ATM Adaptation Layer Type 5) frame is protected and packetloss is prevented. The “sufficient number of cells for transmission ofthe maximum length of packets” is set in advance, based on the designpolicy of the network.

In a third invention, when a virtual connection is to be changed, themulticast function of the ATM switch is employed to transfer cellsthrough both transfer lines before and after the change, for asufficient period of time for transmission of the maximum length ofpackets, and thereafter the virtual connection before the change iseliminated, so that an AAL5 frame is protected and packet loss isprevented. The “sufficient period of time for transmission of themaximum length of packets” is set in advance, based on design policy ofthe network.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an example of a network to which IP switchingis applied;

FIG. 2 is a view showing the simplest construction of the IP switchingnetwork and internal structures of nodes;

FIG. 3 is a view showing how a packet is transferred in a normal state;

FIG. 4 is a view showing how a packet is transferred using a new virtualconnection;

FIG. 5 is a view showing how a packet is transferred using a new virtualconnection, when a packet processing in an IP switch is omitted;

FIG. 6 is a view showing an example of a virtual connection settingrequest;

FIG. 7 is a view showing an example of a virtual connection changerequest;

FIG. 8 is a sequence diagram showing how information is interchanged inFIGS. 4 and 5;

FIG. 9 is a view showing correspondence relations among a packet, anAAL5 frame and ATM cells;

FIG. 10 is a sequence diagram showing an example where a packet is lostowing to a virtual connection change request;

FIG. 11 is a sequence diagram in the case where a waiting time for achange of virtual connections is set;

FIG. 12 is a sequence diagram in the case where multicast period W2 isset;

FIG. 13 is a sequence diagram in the case where multicast period W3 isset;

FIG. 14 is a view showing a basic internal structure of a switch section24;

FIG. 15 is a view showing a basic internal structure of an ATM interfacesection 242;

FIG. 16 is a flowchart showing a virtual connection change procedure;

FIG. 17 is a view showing an internal structure of the ATM interfacesection 242 provided with a PT field monitoring section;

FIG. 18 is a flowchart showing a virtual connection change procedurewhich includes monitoring of a PT field;

FIG. 19 is a view showing an example of rewriting an output-side portidentifier, using a header transform table;

FIG. 20 is a view showing an internal structure of the ATM interfacesection 242 provided with a cell counting section;

FIG. 21 is a flowchart showing a virtual connection change procedurewhich includes counting of cells;

FIG. 22 is a view showing an example of rewriting a head transform tablein the case of using the multicast system;

FIG. 23 is a view showing an internal structure of the ATM interfacesection 242 provided with a timer section;

FIG. 24 is a flowchart showing a virtual connection change procedureusing the timer section;

FIG. 25 is a view showing an internal structure of the switch sectionprovided with the timer section;

FIG. 26 is a flowchart showing a virtual connection change procedure inthe case where the switch section is provided with the timer section;and

FIG. 27 is a view showing an internal structure of the ATM interfacesection 242.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a view showing an example of a network to which IP switchingis applied. The network comprises nodes connected by communicationmedia. The nodes include IP switches, edge nodes of the IP switchingnetwork (terminals or IP gateways), non-IP switching nodes (terminals orIP routers), and the like. An IP switch is a router employing the IPswitching technique, and is connected with other IP switches and edgenodes of the IP switching network so as to form the network. Among theedge nodes of the IP switching network, one situated at the border witha non-IP switching network to communicate therewith is called an IPswitch (IP gateway).

Next, the principle of the IP switching will be described. FIG. 2 showsthe simplest construction of the IP switching network. The node 1A andthe node 1B are edge nodes of the IP switching network, and areconnected with each other through the node 2 which is an IP switch. Thenode 1A is a sending node, and comprises a packet processing section 11Awhich performs transfer processing on the IP level, and a packet-to-celldisassembly section 12A which disassembles a packet into ATM cells andperforms transfer processing in ATM. The node 1B is a receiving node,and comprises a packet processing section 11B which performs transferprocessing on the IP level, and a cell-to-packet assembly section 12Bwhich assembles cells received in ATM to restore them to a packet. Thenode 2, which is an IP switch, comprises a packet processing section 21which performs transfer processing on the IP level, a cell-to-packetassembly section 22 which assembles cells received in ATM to restorethem to a packet, a packet-to-cell disassembly section 23 whichdisassembles a packet to ATM cells to transfer them in ATM, and a switchsection 24 which performs switching on the ATM level. The switch section24 has ports P1-P4, which are connected to the cell-to-packet assemblysection 22, the packet-to-cell disassembly section 23, the node 1A, andthe node 1B, respectively. A virtual connection VC51 is established inadvance between the node 1A and the node 2, and a virtual connectionVC52 is established in advance between the node 2 and the node 1B. Thesevirtual connections are prepared for ordinary data transfer and controlmessage transfer.

Here, for the sake of simplicity, the node 1A is described as having thepacket-to-cell disassembly section only, and the node 1B thecell-to-packet assembly section only. In fact, communication isperformed bilaterally, and each node has both the packet-to-celldisassembly section and the cell-to-packet assembly section. Further,here, the packet-to-cell disassembly section and the cell-to-packetassembly section are described as separate functional blocks. However,it is possible for both sections to be implemented in one functionalblock. In that case, the ports P1 and P2 are reduced to one port.

FIGS. 3-5 are views explaining the principle of IP switching. Thesefigures illustrate a state where packets A, B and C are sent from thenode 1A to the node 1B consecutively.

At first, FIG. 3 is referred to. Here, a packet A is generated in thenode 1A. Then, in the first place, the packet processing section 11Ajudges that the packet A is one destined for the node 1B, decides totransfer the packet toward the node 2, the adjacent node in thedirection of the node 1B, and delivers the packet A to thepacket-to-cell disassembly section 12A. The packet-to-cell disassemblysection 12A disassembles the packet A into ATM cells A1-An, andtransfers them through the virtual connection VC51. The switch section24 receives the ATM cells A1-An, and transfers them to thecell-to-packet assembly section 22. On receiving the ATM cells A1-An,the cell-to-packet assembly section 22 assembles them into the packet Aand sends it to the packet processing section 21. The packet processingsection 21 judges from the contents of the packet A that the packet Ashould be sent to the node 1B, and sends it to the packet-to-celldisassembly section 23. The packet-to-cell disassembly section 23disassembles the packet A into the ATM cells A1-An again, and transfersthem to the node 1B through the virtual connection VC52 destined for thenode 1B. Last, the cell-to-packet assembly section 12B of the node 1Breceives the ATM cells A1-An, assembles them into the packet A, andsends it to the packet processing section 11B, completing the transferprocess.

At this time, the packet processing section 21 of the node 2, which hasprocessed the packet A, estimates whether another packet is to bereceived hereafter consecutively, based on the flow rate of cells sentfrom the node 1 for a given period of time. When it is estimated thatanother packet is to be received consecutively, the packet processingsection 21 sends a request G1 to the switch section 24 for setting a newvirtual connection VC53 as shown in FIG. 4. At the same time, the packetprocessing section 21 sends a request I1 to the node 1A for sendingfollowing packets through the virtual connection VC53. In this example,the request I1 is sent through a control line between the node 1A andnode 2. It, however, may be sent through the virtual connection VC51.

FIG. 6 shows an example of the request G1 for setting a new virtualconnection from the packet processing section to the switch section 24.The reference symbol G11 refers to a message type (in this case, “Makenew connection”), and G12-G15 refer to information elements for settinga virtual connection in the switch section 24. Namely, G12 refers to aninput-side port identifier, G13 to an input-side virtual connectionidentifier x representing the virtual connection VC53 to be set newly,G14 to an output-side port identifier, and G15 to an output-side virtualconnection identifier y representing the virtual connection VC53 to benewly set. Thus, the example shown in FIG. 6 represents a request, “Seta new virtual connection VC53 which connects the virtual connectionidentifier x at the input-side port P3 with the virtual connectionidentifier y at the output-side port P1”.

Next, FIG. 4 will be referred to. In FIG. 4, a packet B generated in thenode 1A is transferred to the node 1B as in FIG. 3. The difference fromFIG. 3 lies in that the ATM transfer from the node 1A to the node 2 isperformed through the virtual connection VC53.

In parallel with this, similarly to the node 2 in FIG. 3, the node 1Bestimates from the former packet A that another packet is to be receivedconsecutively hereafter, and sends the node 2 a request I12 for a newvirtual connection for transmission. On receiving the request, insteadof making a new virtual connection, the node 2 sends a virtualconnection change request G2 to the switch section 24 for making avirtual connection between the node 1B and the node 2 corresponding tothe virtual connection VC53. As a result, the virtual connection VC53 ischanged to a virtual connection VC53′ shown in FIG. 5.

FIG. 7 shows an example of the virtual connection change request G2 sentfrom the packet processing section 21 to the switch section 24.Similarly to G1, G21 refers to a message type (in this case, “changeconnections”), and G22-G27 refer to information elements for setting avirtual connection in the switch section 24. Namely, G22 refers to aninput-side port identifier, G23 to an input-side virtual connectionidentifier x representing the virtual connection VC53′ to be set newly,G24 to an output-side port identifier before change, G25 to anoutput-side virtual connection identifier y before change, G26 to anoutput-side port identifier after change, and G27 to an output-sidevirtual connection identifier z after change representing the virtualconnection VC53′ to be newly set. From G22-G25, the switch section 24judges which virtual connection is requested, and replaces only theoutput-side setting with the information in G26 and G27.

Thus, the example shown in FIG. 7 represents a request, “Change thevirtual connection VC53, which is represented by the input-side portP3—virtual connection identifier x, and the output-side port P1—virtualconnection identifier y, to the virtual connection VC53′ which isrepresented by the input-side port P3—virtual connection identifier x,and the output-side port P4—virtual connection identifier z”.

Last, FIG. 5 will be referred to. As shown in FIG. 5, a packet C istransferred from the node 1A to the node 1B through the virtualconnection VC53′ established by the above process. Here, in the node 2,the packet processing in the network layer handled by software isdispensed with, and only hardware processing in the ATM layer isperformed, and accordingly the packet can be transferred at higherspeed.

FIG. 8 is a sequence diagram showing how the information in FIGS. 3, 4and 5 is interchanged. As shown in FIG. 8, the packet processing section21 sends the switch section 24 the virtual connection change request G2.In fact, since the switch section 24 and the packet processing section21 are not synchronized, there may arise a case that, during theprocessing in the switch section 24, a virtual connection change requestrelated to an already-processed packet arrives from the packetprocessing section 21.

For example, a sequence diagram of FIG. 10 shows an example where, whilethe switch section 24 is processing the cell string C1-Cn which isobtained by disassembling the packet C, the switch section 24 receives avirtual connection change request G2 from the packet processing section21. Here, in FIG. 10, after the switch section 24 has completedprocessing of the cell C1 disassembled from the packet C, the virtualconnection request G2 arrives at the switch section 24 from the packetprocessing section 21. As a result, the virtual connection VC53 whichhas been destined for the cell-to-packet transform section 22 of thenode 2 is changed to one destined for the cell-to-packet transformsection 12B of the node 1B. As a result the cell C1 arrives at thecell-to-packet transform section 22 of the node 2, and the cells C2-Cnarrive at the cell-to-packet transform section 12B of the node 1B. Ineach of the cell-to-packet transform sections 22 and 12B, the set ofcells is not complete, and is discarded in that section. As a result,this packet C is lost on the network, and communication between the node1A and the node 1B is not performed normally.

As a system for packet communication in ATM, a system called AAL5 (ATMAdaptation Layer Type 5) is generally used, and IP switching alsoemploys this system. Procedures in this system will be describedreferring to FIG. 9.

First, on a sending side, a trailer 62 is added to a packet 61 in thenetwork layer to obtain an AAL5 frame. The obtained AAL5 frame isdisassembled into pieces of ATM data 65. Each piece of the ATM data 65is provided with an ATM header 63 to be an ATM cell, which is sent ontoa virtual connection in ATM. An ATM header contains a part called a PT(Payload Type) field 64. For an ATM header 63 of a cell located at theend of an AAL5 frame, a value “1” is set to its PT field 64, and for theother cells, a value “0” is set to their respective PT fields 64, thusindicating a boundary of the frame. On a receiving side, a cell with thevalue “1” in the PT field 64 is detected to separate a string of cells,and the trailers 62 are checked to confirm that the frame is not broken.Then, after removing the trailers 62, the frame is delivered in the formof the packet 61 to the network layer.

FIG. 11 is a sequence diagram showing basic operation of the presentinvention. Similarly to FIG. 10, here also, a virtual connection changerequest G2 arrives after C1 has been processed. As shown in FIG. 11 by awaiting time W1 for a change, the switch section 24 awaits arrival ofthe cell Cn having the PT field value “1” before changing the virtualconnection VC53. This waiting time W1 is set as a sufficient period oftime for sending the largest packet allowed by the network. Variousvalues may be set to W1 depending on the design policy of the network.As a result, no packet is lost in the communication between the node 1Aand the node 1B, and communication is performed normally. In this FIG.11, although the order of the packet C and a packet D is reversed, itcan be restored by processing in the network layer. Differently from theloss, it is not necessary to await timeout, and hardly any qualityproblem arises.

FIG. 14 shows an internal structure of the switch section 24. A cellfrom the outside is received at an ATM interface section 242 on an inputside, its header is exchanged, and thereafter, the cell is directed by aswitch fabric section 241 toward an objective direction, and outputtedthrough an ATM interface section 242 on an output side. A switch controlinstruction from the packet processing section 21 is temporarily sent toa switch control section 243 through an ATM interface section 242. Inthe switch control section 243, the switch control instruction istransformed into an interface control instruction, and sent to an ATMinterface section 242 through an internal line 244, to be processed.

FIG. 15 shows the structure of an ATM interface section 242. A cellinputted into the ATM interface section 242, is temporarily stored in acell buffer 2421, and transformed in a header transform section 2422,and sent to a switch fabric section 2423. The interface controlinstruction from the switch control section 243 is received by aninterface control section 241 and executed there.

FIG. 16 is a flowchart showing an example of a procedure for changingvirtual connections. First, when the packet processing section 21 judgesthat a change of switch setting is necessary, the packet processingsection 21 sends a virtual connection change request to switch section24 (Step S1). This request is received by the switch control section 243within the switch section 24 (Step S2), and the switch control section243 transforms the request into an interface control instruction andsends it to the interface section 242 (Step S3). The interface controlinstruction is received by the interface control section 2423 within theATM interface section 242 (Step S4), and the interface control section2423 rewrites a table in the header transform section 2422 (Step S5),completing the change of the virtual connections.

FIG. 19 shows an example of rewriting a header transform table. This isan example of a header transform table for the port P3, including pairsof an input-side virtual connection identifier and output-side portidentifier—virtual connection identifier. In FIG. 19, entries 71 and 72correspond to the change from the virtual connection VC53 to the virtualconnection VC53′ in FIGS. 4, 5, and 7. Namely, the entry 71 representsthe virtual connection VC53, and the entry 72 the virtual connectionVC53′.

FIG. 17 illustrates an internal structure of the ATM interface section242 on the input side, according to the present invention. In comparisonwith FIG. 15, a PT field monitoring section 2424 is newly added.

FIG. 18 is a flowchart showing an example of a procedure for changingvirtual connections using the PT field monitoring section. Between thestep S4 and the Step S5 of FIG. 16, Step S6 and Step S7 are inserted. InStep 6, the ATM interface control section 2423 asks the PT fieldmonitoring section 2424 if a cell with PT field value “1” has beenprocessed. In Step 7, based on the response from the PT field monitoringsection 2424, the interface control section 2423 judges if to proceed toStep 5.

Rewriting of the header transform table performed in Step 5 is similarto FIG. 19.

Further, the PT field monitoring section 2424 may be provided with afunction of investigating a cell flow and a timer function, by replacingit with a PT field monitoring section 2424 a and a cell flow monitoringsection 2424 b (FIG. 27). By this construction, in the case that avirtual connection change request arrives between packets, if the cellcounting section 2424 b does not count a cell flow for a given period oftime, it is possible to change virtual connections without awaitingprocessing of a subsequent packet.

FIG. 12 is a sequence diagram showing basic operation of the presentinvention. The packet D shown in FIG. 12 is a packet sent following thepacket C. Here also, similarly to FIG. 10, a virtual connection changerequest G2 arrives after a cell C1 has been processed. The switchsection 24 resets the virtual connection VC53, which, at this point intime, is destined for the cell-to-packet assembly section 22 of the node2, so that the virtual connection VC53 is destined for thecell-to-packet assembly section 12B of the node 1B, in addition to thecell-to-packet assembly section 22, by a multicast function. Theoperation can be seen from FIG. 12 by noting a multicast period W2.Namely, after completing transmission of a sufficient number (n in thisexample) of cells for the maximum length of packets, setting of thedestination to the cell-to-packet assembly section 22 is eliminated inthe virtual connection VC53, leaving only the virtual connection VC53′destined for the cell-to-packet section 12B. Although the packet D isdiscarded in the cell-to-packet assembly section 22 once, and the packetC is discarded in the cell-to-packet assembly section 12B once, both thepacket C and packet D arrive at the node 1B in the result. Thus, thecommunication is performed normally on the packet level.

FIG. 20 is a view illustrating an internal structure of the ATMinterface section 242 on the input side, according to the presentinvention. It differs from FIG. 17 in that the PT field monitoringsection 2424 is replaced by a cell counting section 2425.

FIG. 21 is a flowchart showing an example of a procedure for changingvirtual connections using the cell counting section. It is the same asFIG. 18 up to Step S4. Thereafter, the ATM interface control section2423, which has received the virtual connection change request from theswitch control section 243, rewrites the table of the header transformsection 2422 so that, while maintaining the virtual connection beforethe change, the same cell is also multicasted onto the virtualconnection after the change (Step S8). Then, the ATM interface controlsection 2423 inquires of the cell counting section 2425 about thecurrent number of cells (Step S9). Based on a response from the cellcounting section 2425, the interface control section 2423 judges if agiven number of cells have been processed (Step S10). Judging that thegiven number of cells have been processed, the interface control section2423 eliminates an entry representing the virtual connection before thechange from the table of the header transform section 2422 (Step S11),completing the processing of changing the virtual connections.

FIG. 22 shows an example of rewriting the head transform table in themulticast system. It differs from FIG. 19 in that there exists a periodof multicast processing as shown by an entry 73. Some cells disassembledfrom the packet C generated in the sending node 1A arrive at thereceiving node 1B in duplicate. A larger number of the duplicate cellsis preferred for preventing packet loss and retransmission due to packetloss. On the other hand, from the viewpoint of packet processing in theIP switch and a transfer rate of the network, it is more desirable if asmaller number of cells are transferred in duplicate, i.e., multicasted.

To obtain the above-described given number for cells, the maximum lengthof packets generally used in an IP network system, i.e., 1500 byte, maybe taken into consideration. This number 1500 is divided by 48 bytes,the length of the payload of the ATM cell, raised to a unit, and addedwith the length of 1 cell of the AAL5 trailer, giving 32, which may beconsidered an effective value of the given number for cells. Further,when, 65,536 bytes, the maximum length of packets in AAL5 is taken,1,367 may be considered as the effective value.

FIG. 13 is basically similar to FIG. 12 except that a multicast periodW3 is fixed as a given period of time, with its length not beingdynamically changed depending on the number of cells.

FIG. 23 is a view illustrating an internal structure of the ATMinterface section 242 on the input side, according to the presentinvention. Differently from FIGS. 17 and 20, a timer section 2426 isprovided, and the cell buffer is not monitored.

FIG. 24 is a flowchart showing an example of a procedure for changingvirtual connections in the present embodiment. It is similar to FIG. 21except that the timer section 2426 is started up, instead of the cellcounting section 2425 (Step S12), and that, awaiting the timerexpiration, which is notified by a signal from the timer section 2426(Step 13), the virtual connection before the change is eliminated.

Rewriting of the header transform table in the present embodiment issimilar to FIG. 22 which is the example for the embodiment 2.

FIG. 25 shows an internal structure of the switch section 24, accordingto the present invention. Differently from the embodiments describedheretofore, a timer section 245 is connected to the switch controlsection 243. By this construction, it is not necessary to add a newstructure to the ATM interface section 242.

FIG. 26 is a flowchart showing an example of a procedure for changingvirtual connections. It is the same as the above-described flowcharts upto Step S2. Thereafter, first, the switch control section 243, which hasreceived a virtual connection change request, sends a request forsetting a virtual connection after the change to the ATM interfacesection 242 (Step S14). On receiving that request, the interface controlsection 2423 within the interface section 242 rewrites the table of theheader transform section 242 in accordance with the request. As aresult, virtual connections are generated for multicasting both to thevirtual connection before the change and to the virtual connection afterthe change (Step S15). Thereafter, the switch control section 243 startsup the timer section 245 (Step S16), and awaits a timer expirationsignal (Step S17). When the timer expires, then, the switch controlsection 243 sends a request for eliminating the virtual connectionbefore the change to the ATM interface section (Step S18). On receivingthe request, the interface control section 2423 within the interfacesection eliminates the entry related to the virtual connection beforethe change from the table of the header transform section 2422 (StepS19), completing the process of changing the virtual connections.

Rewriting of the header transform table in the present invention issimilar to FIG. 22 which is the example for the embodiments 2 and 3-1.

According to the first invention, it is possible to prevent packet lossat the time of changing virtual connections. Advantageously, at thattime, it is not necessary to transfer useless cells.

According to the second invention, it is possible to prevent packet lossat the time of changing virtual connections. Advantageously, in the casethat the ATM switch supports the multicast function and the ATMinterface section is provided with the cell counting section in advance,the invention can be implemented without addition of special hardware.

According to the third invention, it is possible to prevent packet lossat the time of changing virtual connections. Advantageously, this systemdoes not require any circuit for high speed processing such as searchingof the cell buffer's contents, and has a high degree of freedom in itsconstruction.

What is claimed is:
 1. A switching device comprising: a receiving port which receives cells; a switch which switches cells received at the receiving port; and a sending port which sends cells which are switched by the switch, wherein the switch which sets a first virtual connection for transferring the received cells through the sending port after packet processing of the received cells, and which sets a second virtual connection for transferring the received cells to the sending port; and when the first virtual connection is to be changed to the second virtual connection in response to a request received from an external side of the switching device, the switch keeps the first virtual connection until cells comprising at least one packet of data have been sent.
 2. The switching device according to claim 1, wherein the switch device further comprises: a cell counter which counts a number of cells received after receiving the request; and a header transform section which transforms a header of a cell received, for changing virtual connection for transmitting cells from the first virtual connection to the second virtual connection, after the number of cells counted by the cell counter reaches a predetermined number.
 3. The switching device according to claim 1, wherein the switch device further comprises: a timer which measures time until receiving cells comprising a new packet; and a header transform section transforms a header of a cell received, for changing virtual connection for transmitting cells from the first virtual connection to the second virtual connection, after the timer measures a predetermined time.
 4. The switching device according to claim 1, wherein the switch transfers the received cells with the first virtual connection and the second virtual connection until cells comprising at least one packet of data have been sent.
 5. A switching device according to claim 1, wherein the cells are ATM cells.
 6. A switching device comprising: a receiving port which receives cells; a switch which switches cells received at the receiving port; a cell-to-packet transform section which receives, through the swicth section, the cells received by the receiving port, and assembles the cells into a packet; a packet processing section which processes the packet transformed from the cells by the cell-to-packet transform section; a packet-to-cell transform section which disassembles the packet processed by the packet processing section into cells and sends the cells to the switch section; and a sending port which receives the cells sent from the packet-to-cell transform section through the switch, and sends the cells; wherein the switch which sets a first virtual connection which leads through the cell-to-packet transform section, the packet processing section and the packet-to-cell transform section for transferring the received cells through the sending port, and which sets a second virtual connection which leads through the switch for transferring the received cells to the sending port; wherein the switch transfers the received cells with the first virtual connection and the second virtual connection until cells comprising at least one packet of data have been sent, when the switch receives a request received from an external side of the switching device to change from the first virtual connection to the second virtual connection.
 7. The switching device according to claim 6, wherein the switch device further comprises: a cell counter which counts a number of cells received after receiving the request; and a header transform section which transforms a header of a cell received, for changing virtual connection for transmitting cells from the first virtual connection to the second virtual connection, after the number of cells counted by the cell counter reaches a predetermined number.
 8. The switching device according to claim 6, wherein the switch device further comprises: a timer which measures time until receiving cells comprising a new packet; and a header transform section transforms a header of a cell received, for changing virtual connection for transmitting cells from the first virtual connection to the second virtual connection, after the timer measures a predetermined time.
 9. A switching device according to claim 6, wherein the cells are ATM cells. 